1. Field of the Invention
The present invention relates to a magnetic tape cassette, and more particularly to a magnetic tape cassette for use in effecting recording and reproduction by means of digital signals.
2. Related Art
Conventionally, magnetic tape cassettes have been used for PCM recording/reproducing apparatuses for converting analog information such as acoustic signals into digital signals, e.g., PCM signals, so as to record and/or reproduce the same on magnetic tape. In this magnetic tape cassette, since it is compact and lightweight, and high-density recording is effected, a structure is particularly adopted for preventing fingers from coming into contact the magnetic tape or dust from attaching to the same.
That is, if fingers come into contact with the magnetic tape or dust attaches to the same, a dropout occurs in a reproduced signal. Hence, in order to prevent this, it is necessary to hermetically seal the magnetic tape in a cassette case as much as possible such as during storage of the cassette. Meanwhile, when the cassette is used, insertion holes for reel shafts and a space for inserting guides and the like during recording and/or reproduction are naturally required in the cassette case. As a structure which is adopted in view of such a need, a lid for opening or closing a front portion of the cassette and a slider which slides in a transverse direction of the cassette in contact with the bottom of the cassette are provided.
As a conventional cassette case having such a function, a DAT cassette having a mechanism for locking a slider 3 is known, as shown in FIGS. 1 and 2. In this cassette, a resiliently locking piece 2 is a cantilevered arm formed in a bottom of a lower half 1b in the form of a cutout, and has a smaller thickness than the lower half 1b, and a head 2a adapted to be fitted in engaging holes 3a and 3b provided in a guiding shallow groove 3c of the slider 3 is formed at a distal end thereof.
In addition, center lines of the resiliently locking piece 2 and the guiding shallow groove 3c of the slider 3 in the transverse direction of the cassette overlap with each other. FIG. 1 shows a state in which the head 2a of the resiliently locking piece 2 is fitted in the rear engaging hole 3b in the slider 3 (i.e., a state in which the aforementioned space is closed by the slider).
A spring-receiving projection 4 is formed on the inner side of the head of the resiliently locking piece 2, while a projection 5 for fitting a spring at a position corresponding to this projection 4 is formed on the inner side of an upper half la. A compression coil spring 6 is interposed between these two projections so as to urge the resiliently locking piece 2 toward the slider 3 (toward the lower side).
However, with the above-described conventional arrangement, in order to avoid interference with the tape and the like accommodated in the cassette case, the coil diameter of the compression coil spring 6 is formed to be small as compared with its free-length direction (extending and contracting direction), and the compression coil spring 6 tends to be used as a relatively elongated compression spring. As a result, there is no problem if the compression coil spring 6, when compressed, operates such as to contract positively in the compressing direction, but there are cases where it is deformed such as to be twisted in the radial direction of its coil (in the lateral direction). Such unexpected behavior of the compression coil spring causes a lack in the stability of spring pressure, so that its function becomes impaired.
Meanwhile, if the resiliently locking piece 2 is provided with a structure for restricting its movement in the inward direction of the cassette, it is possible to contribute to the stabilization of the function of the resiliently locking piece. However, although it is desirable to provide this restricting effect at the distal end side of the resiliently locking piece 2, if a structure is adopted in which this restricting member is provided at the distal end or in its vicinity, there has been a problem in that the restricting member interferes with the compression coil spring or imparts a restriction to the arrangement of the compression coil spring.
There have been known the other type of conventional cassette case having such a function, those disclosed in, for instance, Unexamined Japanese Utility Model Application (OPI) Nos. Sho. 61-121580 and Sho. 62-147188, and Japanese Patent Publication No. Sho. 62-189687.
The one disclosed in Unexamined Japanese Utility Model Application No. Sho. 61-121580 is structured such that a limiter consisting of a large projection jutting out in the inward direction of the cassette is provided at a distal end of a resiliently locking piece, and this limiter is made to abut against an inner surface of an upper half so as to restrict an amount of deformation of the resiliently locking piece.
The one disclosed in Unexamined Japanese Utility Model Application No. Sho. 62-147188 is arranged such that a limiter member is disposed on the upper half or the resiliently locking piece in correspondence with the inner side of the resiliently locking piece to restrict an amount of deformation of the resiliently locking piece by means of the limiter member, and a wall portion surrounding the limiter member is provided.
As shown in FIG. 3, the one disclosed in Japanese Patent Publication No. Sho. 62-189687 is arranged such that a resiliently locking piece 2 is a cantilever formed in a bottom of a lower half 1b in the form of a cutout, and has a thickness smaller than the lower half 1b. A head 2a capable of fitting in engaging holes 3a, 3b formed in a shallow groove 3c of a slider 3 is provided at a distal end of the resiliently locking piece 2. A spring-receiving projection 4' is formed on the inner side of the head of the resiliently locking piece 2, while a projection 5' is formed on the inner side of an upper half la for fitting a spring at a position corresponding to this projection 4'. A coil spring 6 is interposed between these projections so as to urge the resiliently locking piece 2 toward the slider 3. FIG. 3 shows a cross-sectional view taken along the resiliently locking piece 2 in a transverse direction of the cassette, and illustrates a state in which the head 2a of the resiliently locking piece 2 is fitted in the rear engaging hole 3b in the slider 3.
However, of the above-described conventional examples, in the structure disclosed in the afore-mentioned Unexamined Japanese Utility Model Application No. Sho. 61-121580, the provision of a limiter, which is a long and large projection as compared with the thin-walled resiliently locking piece, at the distal end Of the locking piece is fraught with problems in that the flow of a resin in a cavity during molding lacks smoothness, thereby causing a decline in moldability. In addition, in the case of the arrangement disclosed in the aforementioned Unexamined Japanese Utility Model Application No. Sho. 62-147188, in a case where a long pin-shaped projection (limiter) is formed on the upper half side, if the projection becomes high, moldability becomes poor in the same way as described above owing to sink marks and the like on the upper half case. As a result, there has been a high risk of producing the problem that the accuracy in engagement between the limiter and the head of the resiliently locking piece declines, thereby making it impossible to sufficiently attain the original object.
Meanwhile, in the structure in which a coil spring is fitted as in the aforementioned Japanese Patent Publication No. Sho. 62-189687, the formation of a relatively long and large spring-receiving projection in the inward direction of the cassette in a projecting manner is advantage in the fitting of the spring and the like. Also, it is possible to expect the operation of the aforementioned limiter by causing this spring-receiving projection to project even further. However, in this case as well, not only does the moldability of the upper half case decline, but also there is the risk of producing unexpected engagement between the spring-receiving projection and the coil spring (e.g., engagement in which a tip of the spring-receiving projection is caught by the coil spring) in the case where the spring-receiving projection is made to jut out on a large scale. Thus, a portion of the coil spring which effectively operates as a spring may possibly change, so that there has been the risk of rendering the spring function unstable.